1. Field of the Invention
The present invention is a storage device, and more particularly a hybrid density memory storage device and a control method thereof.
2. Description of Related Art
Non-volatile memory is used to store data and is always applied to storage devices, such as memory cards, USB flash drives or solid state disk drives. Flash memory has the advantages of high storage density, low power consumption, effective access efficiency and reasonable price, so that it has become the main stream.
The general non-volatile memory usually adopts MLC (Multi-Level Cell) memory or SLC (Single-Level-Cell) memory, wherein the memory made by MLC is high density memory and the memory made by SLC is low density memory. Compared with the low density memory, the data storage capacity per unit area of the high density memory can be multiple, so that the high density memory leads to highly improved storage capacity and reduced cost. However, the time for reading/writing data, executing burning and erasing becomes longer. Besides, the manufacturing technology of the MLC also makes the high density memory to bear fewer erase cycles, so as to influence the data access speed and life time of the storage device with high density memory.
Since the characteristics of the high density memory are high storage capacity and low cost but slow speed for accessing data and fewer erase cycles, and the characteristics of the low density memory are fast speed for accessing data and more erase cycles but small storage capacity and high cost, a memory simultaneously possesses two kinds of densities is developed, namely, a hybrid density memory.
Currently, the hybrid density memory storage device uses low density memory to record data which is often used and high density memory to record large amounts of data. Thus, it has become an issue of research how to simply and rapidly recognize the property of writing data from the host so as to direct the data to be stored in the proper memory. Furthermore, since the low density memory has the characteristics of higher cost and smaller storage capacity, it is also a problem how to plan a data processing method effectively such that the low density memory stores the data which is often used and the older data which is rarely used is eliminated as far as possible. Finally, because the erase cycles of memories in different densities are different, and when the data stored in the memory is updated or accessed, the blocks will be erased which might cause non-average erasing numbers, it will lead to the problem that one memory reaches the erase limit earlier than the other memory, so that the storage device prematurely becomes unusable.